Bearing in mind the increasing importance of plant constituents as renewable resources, biotechnology research attempts to adapt plant raw materials to the demands of the processing industry. Thus, to make possible the use of renewable resources in as many fields of application as possible, it is necessary to make available a great variety of materials.
Not only oils, fats and proteins, but also polysaccharides, constitute important renewable resources from plants. A pivotal position in the polysaccharides is taken up not only by cellulose, but also by starch, which is one of the most important storage substances in higher plants. Not only corn, rice and wheat, but also potato, plays an important role, in particular in starch production.
The polysaccharide starch is a polymer of chemically uniform units, the glucose molecules. However, it is of a highly complex mixture of different forms of molecules which differ with regard to their degree of polymerization and the occurrence of branchings in the glucose chains. Starch is therefore no uniform raw material. In particular, we differentiate between amylose starch, an essentially unbranched polymer of α-1,4-glycosidically linked glucose molecules, and amylopectin starch which, in turn, constitutes a complex mixture of differently branched glucose chains. Other branchings are generated by the occurrence of additional α-1,6-glycosidic linkages. In typical plants used for starch production such as, for example, corn or potatoes, the synthesized starch consists to approx. 25% of amylose starch and approx. 75% of amylopectin starch.
The molecular structure of the starch, which is largely determined by the degree of branching, the amylose/amylopectin ratio, the average length and distribution of the side chains and the presence of phosphate groups is of prime importance for important functional properties of the starch, resp., its aqueous solutions. Important functional properties which must be mentioned are, for example, solubility, retrogradation behavior, film-forming properties, viscosity, color stability, the gelatination properties and the binding and adhesion properties. Also, starch granule size may be of importance for various applications. The production of high-amylose starches is also of particular interest for certain applications. Furthermore, a modified starch contained in plant cells may advantageously alter the behavior of the plant cells under certain conditions. For example, a reduced starch degradation during the storage of starch-containing organs such as, for example, seeds or tubers, before their processing, for example for starch extraction, is feasible. It is also of interest to produce modified starches which make plant cells or plant organs which contain this starch better suited for processing, for example in the preparation of foodstuffs such as popcorn or cornflakes from corn or of potato chips, potato crisps or potato powder from potatoes. Of particular interest is the improvement of the starches regarding a reduced cold sweetening, i.e. a reduced liberation of reducing sugars (in particular glucose) upon prolonged storage at low temperatures. Potatoes especially are frequently stored at temperatures from 4 to 8° C. to minimize starch degradation during storage. The reducing sugars liberated during this process, in particular glucose, result in undesired browning reactions in the production of potato chips or potato crisps (so-called Maillard reactions).
The starch which can be isolated from plants is frequently adapted to particular industrial purposes with the aid of chemical modifications which, as a rule, require time and money. It seems therefore desirable to find possibilities of generating plants which synthesize a starch whose properties already meet the specific demands of the processing industry and thus combine economical and ecological advantages.
A possibility of providing such plants is, in addition to breeding measures, the direct genetic alteration of the starch metabolism of starch-producing plants by genetic engineering methods. However, a prerequisite therefor is the identification and characterization of the enzymes which participate in starch synthesis modification and starch degradation (starch metabolism) and isolation of the corresponding DNA sequences which encode these enzymes.
The biochemical pathways which lead to the synthesis of starch are essentially known. In plant cells, starch synthesis takes place in the plastids. In photosynthetically active tissues, these plastids are the chloroplasts, in photosynthetically inactive, starch-storing tissues the amyloplasts.
Important enzymes which participate in starch synthesis are, for example, the branching enzymes, ADP glucose pyrophosphorylases, granule-bound starch synthases, soluble starch synthases, debranching enzymes, disproportioning enzymes, plastidic starch phosphorylases and the R1 enzymes (R1 proteins).